Jim Carlton

Devlin Barrett

Miguel Bustillo

SAN BERNARDINO, Calif.—Authorities said Thursday that the two suspects who stormed a holiday gathering of county employees on Wednesday, killing 14 people, deployed remote-controlled pipe bombs at the scene and had amassed thousands of rounds of ammunition.

Law-enforcement officials said they were still unsure of what motivated Syed Rizwan Farook and Tashfeen Malik, husband and wife, to open fire at the event for county employees while dressed in “assault-style clothing” and armed with semiautomatic weapons.